Their eye is shut!?

I have a hen and a rooster that have one eye shut...it looks alittle swollen but no other problems. They are eating and drinking, pooing, mating, etc. We just noticed this, this morning. What is it? Anyone have any idea why their eye would be shut and swollen? Like I said, they are acting completely normal.

Watch it overnight. They could have just gotten pecked in the eye by another chicken. If not, then they might have an infection. I would just watch their eyes for now and make sure nothing nasty is coming out, like pus. Good luck!